<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #box > p{
            color: blue;
        }
    </style>
</head>
<body>
<div id="box">
    <p>hello world</p>
    <p>hello world</p>
    <p>hello world</p>
    <span>123</span>
    <div class="demo">
        <p>hello world1213</p>
        <p>hello world1231</p>
        <p>hello world123</p>
    </div>
</div>
<p>hello world</p>
<p>hello world</p>
<p>hello world</p>
</body>
</html>
<script src="js/jquery-1.10.1.js"></script>
<script>
    $(function () {
        $("#box > p").css("color","purple")
    })
</script>